1. Field of the Invention
This invention relates generally to a vehicle powertrain applicable to a hybrid electric vehicle (HEV). More particularly, the invention pertains to active use of wheel brakes in the control of an engine restart.
2. Description of the Prior Art
A HEV is a vehicle that combines a conventional propulsion system, which includes an internal combustion engine and a step-change automatic transmission, a rechargeable energy storage system that includes an electric motor and electric storage battery to improve fuel economy over a conventional vehicle.
Motor vehicles can be designed to employ certain aspects of hybrid electric technology, but without use of a hybrid electric powertrain. Certain vehicles having a conventional powertrain but no electric machine for driving the wheels, called micro-HEVs, shutdown the engine at idle speed to reduce fuel consumption and reduce emissions while the vehicle is stopped.
During normal vehicle operation many instances arise where the vehicle must stop: at traffic signals, cross-walks, stop signs and the like. In micro-HEVs, the engine is shut down if no power is required, e.g. while waiting at a traffic light. As soon as power is requested, the engine is automatically restarted. By avoiding an unnecessary engine idling event, the vehicle's fuel economy is improved. For this purpose, it is desirable to shut down the engine function as much as possible when certain engine stop conditions are satisfied.
In a HEV equipped with an automatic transmission, a powertrain torque spike accompanies an engine startup process and occurs almost in phase with the engine speed increase. This peak powertrain torque disturbance is generated by the torque converter during the engine restart event and is transferred to the wheels if the engine is restarted in gear. This peak powertrain torque disturbance may produce as much as 0.26 g forward acceleration pulse in the vehicle producing an uncomfortable jerk.
Another torque disturbance that occurs during an engine restart in-gear is the gradient load torque TRL, which is transmitted to the wheels by the driveline. TRL=mg sin θ, wherein (m) is vehicle mass and (θ) is the road gradient angle.
In a passive brake-based wheel torque disturbance suppression control strategy, the brake master cylinder pressure threshold, which enables the engine shutdown action, must be increased in order to ensure the satisfaction of the passive brake pressure control operation. Unfortunately, fuel economy is jeopardized by the increased brake pressure threshold because less engine shutdown action is executed. Passive brake-based wheel torque disturbance suppression control performance is substantially affected by the operator's driving habits because hard braking is required to actuate the engine shutdown function. The consistency of the engine stop/start function, as well as the resulted fuel economy rating, will also change from driver-to-driver.
A need exists in the industry for a technique during an engine stop and restart event in a micro-HEV that suppresses the torque surge and keeps the vehicle in standstill when the engine is restarted during vehicle launch and during system triggered engine automatic startup.